A-Level Physics: Thermodynamic Systems Guide
Download printable cheat-sheet (CC-BY 4.0)14 Jul 2025, 00:00 Z
TL;DR
Thermodynamics is the energy balance sheet of the universe. Nail the distinctions between U, Q, W and you will unlock marks in gas laws, kinetic theory and every practical calorimetry task.
1 Internal energy \((U)\)
The macroscopic state of a system fixes its internal energy: the sum of random microscopic kinetic \((E_k)\) and potential \((E_p)\) energies. No single thermometer can reveal \(U\) directly — you must track how energy enters or leaves.
1.1 Parent takeaway
A student who writes “heat stored in the gas” is forfeiting method marks. Heat is a transfer, not a store.
1.2 Mini-drill
State whether each contributes to \(U\):
| Scenario | Contributes to \(U\)? |
| :- | :- |
| Translational motion of the whole cylinder | No |
| Vibrations between gas molecules | Yes |
2 Thermodynamic temperature \((T)\)
Absolute temperature in kelvin is directly proportional to the mean kinetic energy per particle. Doubling \(T\) doubles \(\langle E_k \rangle\). Always quote exam answers to 2-3 s.f. unless otherwise stated.
3 Heating \((Q)\) vs work \((W)\)
| Symbol | Process | Positive when... | Typical formula |
| :-: | :- | :- | :- |
| \(Q\) | Energy transfer by heating | Energy enters system | \(Q = mc \Delta T\) |
| \(W\) | Mechanical work | Done on system | \(W = p \Delta V\) (constant \(p\)) |
Sign hack: In SEAB mark schemes, work done by the gas is negative.
4 Zeroth law — the thermometer principle
If A is in thermal equilibrium with B, and B with C, then A is in equilibrium with C. This justifies using a third body (a thermometer) to compare temperatures.
5 First law — the energy ledger
\[ \Delta U = Q + W. \]
- Positive \(Q\): heating adds energy.
- Positive \(W\): surroundings compress system.
- For an ideal gas in free expansion, \(Q = 0\) and \(W = 0\), hence \(\Delta U = 0\): temperature unchanged.
6 Specific heat capacity \((c)\)
Defined as the heat required per unit mass per kelvin rise:
\[ Q = mc \Delta T. \]
6.1 Quick-check
Water has \(c \approx 4.18 \space \pu{kJ.kg-1.K-1}\) — roughly 30x that of copper. That is why water is a coolant.
7 Specific latent heat \((L)\)
Energy per unit mass for a phase change at constant \(T\):
\[ Q = mL. \]
Common values:
| Substance | \(L_f\) (fusion) / \(\pu{kJ.kg-1}\) | \(L_v\) (vaporisation) / \(\pu{kJ.kg-1}\) |
| :- | :-: | :-: |
| Water | 334 | 2260 |
8 WA & Paper 4 timing rules
- 1 mark ≈ 1.5 min — budget your section time.
- Copy units first when using data-book tables.
- Show full working; SEAB awards method marks even if arithmetic slips.
9 Further reading
10 Call-to-action
Parents: schedule a 1-hour Thermodynamic Systems clinic two weeks before WA 2.
Students: print the First Law equation and stick it on your water bottle — then apply it to every past-year Qn you meet.
Last updated 14 Jul 2025. Next review: upon release of the 2027 draft syllabus.
